5G network deployments are in a full swing around the world. The initial phase of 5G deployments focuses on eMBB. This puts very strict requirements on uplink capacity and transmission latency, which cannot be completely met with only the use of the 3.5 GHz band. To meet the challenges, ZTE proposes the FDD assisted super TDD (FAST) 5G solution.
3.5 GHz Spectrum Analysis
Spectrum is the core resource for mobile communications. The 5G spectrum is separated into multiple frequency ranges, each with its own characteristics that makes it suitable for the deployment of a certain service. The world's first commercial 5G networks are deployed with the higher frequency bands including 3.5 GHz (3.3-3.8 GHz, n78 band) and millimeter waves, and 2.6 GHz (2.496-2.69 GHz, n41 band). Compared with the main FDD-LTE bands such as 1.8 GHz (band 3), the 3.5 GHz TDD band has higher penetration loss and less available uplink time slots. Therefore, in meeting 5G service requirements, it faces three key challenges: uplink capacity, uplink coverage and transmission latency.
With TDD mode, the uplink and downlink use the same frequency and are allocated different time slots. In China at 3.5 GHz, the uplink/downlink time slot ratio is 3:7, that is, 30% of the time slots is allocated for uplink and 70% of the time slots for downlink. Taking the 100 MHz bandwidth as an example, the equivalent uplink bandwidth is only 30 MHz, which is only 1.5 times that of the 4G single carrier.
The higher the frequency, the greater the space propagation loss and the shorter the coverage distance. For example, the uplink propagation loss in the 3.5 GHz band is 5 dB higher than the 2.1 GHz band. In addition, the higher the frequency, the greater the penetration loss, and the shorter the coverage distance.
With TDD mode, terminals cannot send uplink data while receiving downlink data, which results in an extra latency for uplink. For the 3.5 GHz band with 30% of the time slots used for the uplink, there will be an extra latency of 0 to 2 ms, with an average latency of 0.8 ms. Likewise, in the downlink direction, an extra latency of 0 to 1 ms and an average latency of 0.2 ms.
ZTE FAST to Improve Network Capacity and Coverage
Enhancing 5G uplink performance with lower frequency bands such as 2.1 GHz and 700 MHz has become a major concern for many operators. ZTE FAST 5G solution effectively improves 5G uplink and downlink performance through deep cooperation of FDD and TDD. Low-band FDD spectrum enables wider coverage and incurs no extra transmission latency but offers a lower bandwidth; mid-/high-band TDD spectrum provides a larger bandwidth that is further improved by the application of MIMO technology in both the uplink and downlink but inferior coverage and latency to FDD. For the terminals in the center of the cell (near point), FAST (as shown in Fig. 1) deeply aggregates the bands of FDD and TDD for simultaneous uplink and downlink transmission to achieve large throughput and low latency. For the terminals at the cell edge (far point), FAST enables them to switch to FDD band in uplink for better coverage while maintaining FDD and TDD carrier aggregation in downlink for higher data speed.
Based on the standard carrier aggregation framework which is already widely applied in 4G network, FAST introduces the innovative uplink TDM scheduling scheme to flexibly aggregate and coordinate FDD and TDD in time and frequency domains to enable spectrum to reach its full potential, thus solving the challenges of uplink bandwidth, uplink coverage and transmission latency.
Enhancing 5G Capacity
Assisted by 20 MHz bandwidth of the 2.1 GHz band, FAST allows a 3.5 GHz 5G network to improve a UE's uplink peak throughput and downlink peak throughput by 23% and 28% respectively. In the future, if 50 MHz bandwidth of the 2.1 GHz band is acquired, FAST can enhance the uplink and downlink peak throughput by 58% and 71% respectively.
A 5G terminal generally has up to two uplink transmit channels. For the TDD band, with the use of 2 × 2 MIMO in uplink, the equivalent throughput doubles. However, if a terminal utilizes the conventional uplink carrier aggregation technology to aggregate the FDD and TDD carriers, FDD and TDD can only use one transmit channel respectively, and the TDD uplink 2 × 2 MIMO will be disabled, which results in the capacity loss. FAST addresses this problem with the TDM scheduling scheme that reserves the 2 × 2 MIMO uplink capability for the TDD carrier. To be more specific, during the time of TDD uplink slots, the two uplink transmit channels work in the TDD 2 × 2 MIMO mode, and during the time of TDD downlink slots, the terminal will switch immediately to the FDD band. This scheduling mechanism uses nearly 100% of the uplink time slots without sacrificing the TDD 2 × 2 MIMO capabilities.
Improving 5G Coverage
When 5G is deployed over the 3.5 GHz band, the coverage bottleneck will first appear in the uplink direction whereas the downlink coverage is still acceptable. This 'asymmetry' between uplink and downlink restricts the coverage of 3.5 GHz and restricts the network spectrum efficiency. With FAST, terminals can connect simultaneously to both FDD and TDD carriers. On the cell edge, the terminal continues to benefit from the large TDD capacity in downlink while the uplink transmission will switch to the FDD carriers for better 5G coverage so that 5G services will be expanded beyond the TDD uplink coverage.
The deep harmonization of FDD and TDD provides larger coverage than a single TDD carrier and higher downlink data speeds than a single FDD carrier. Taking 2.1 GHz (FDD) and 3.5 GHz (TDD) as an example, the terminal can switch to 2.1 GHz in uplink when moving beyond the uplink coverage edge of 3.5 GHz, providing 2.3 times more uplink time slots than when there is only a single carrier of 3.5 GHz, and 2.5 times more downlink bandwidth than when there is only a single 2.1 GHz carrier.
Reducing 5G Latency
With FAST, the terminals can be flexibly scheduled to transmit data on FDD and/or TDD carriers. The downlink and uplink time slots are both 100% available without introducing extra schedule-waiting latency, which reduces the transmission latency. Taking uplink for an example, the average uplink transmission latency of the 3.5 GHz TDD single carrier is about 2.2 ms, which can be reduced by 31% to 1.5 ms with FAST.
Flexible, Easy Networking
FAST is the mainstream solution for 5G NR to facilitate the 5G commercialization. First, it supports inter-sector and inter-site TDD/FDD coordination. Unlike other uplink enhancement solutions, FAST does not have the mandatory FDD and TDD co-site requirement for 5G deployment. Second, there is no tight binding between FDD and TDD carriers, that is, an FDD carrier can deeply be aggregated and coordinated with multiple TDD carriers, and vice versa. Each combination of TDD and FDD carriers is established dynamically for a particular terminal.
In the scenario where FDD is not co-sited with TDD or the coverage of FDD and TDD does not completely overlap, the flexible scheduling technologies, including the static codebook and two PUCCH groups, can be enabled by FAST to ease the deployment restriction.
In November 2019, ZTE and China Telecom completed the verification of the industry's first FAST solution at 2.1 GHz and 3.5 GHz, showing that the uplink rate of a single user can be up to 40% higher than that of the 3.5 GHz single carrier. The uplink switching for carrier aggregation proposed by FAST has already been standardized in 3GPP Release 16, and it is expected that this technology will be widely supported by the industry chain to help operators build better networks.